Material with a high degree of impermeability to gases and a method for the production thereof

ABSTRACT

From a general point of view, the present invention relates to a material which has a high degree of impermeability to gases, especially to atmospheric oxygen, and which is particularly suitable for the manufacture of containers for easily oxidizable foods or beverages such as, for example, beer, milk, “soft drinks”, and fruit juices. In particular, the present invention relates to a method for the preparation of polymeric material with a high degree of impermeability to gases, the method comprising the step of subjecting the polymeric material to a high-pressure treatment.

[0001] From a general point of view, the present invention relates to a material which has a high degree of impermeability to gases, especially to atmospheric oxygen, and which is particularly suitable for the manufacture of containers for easily oxidizable foods or beverages, such as, for example, beer, milk, “soft drinks”, and fruit juices.

[0002] Although the use of containers made of plastics material for perishable beverages is desirable for reasons of low container cost, it is greatly constrained back by the fact that the materials normally used, such as polyethylene, polyethylene terephthalate, polypropylene, and multi-layer materials derived therefrom, have a permeability to atmospheric oxygen such as to cause deterioration of the beverage due to oxidation, particularly if it is kept for a long time.

[0003] The oxidizable beverages referred to in particular in the present invention, without wishing to be limited thereby, are beer, diary products and “soft drinks”. The latter term is intended to define, in particular, tea and isotonic beverages or beverages supplemented with vitamins or other organic ingredients such as to render them difficult to preserve.

[0004] Various attempts have in fact been made to overcome the above-mentioned problem. For example, copolymers between polyethylene terephthalate (PET) and resorcinol derivatives have been proposed; these have acceptable impermeability to oxygen and are therefore potentially usable for easily oxidizable beverages (Resorcinol Chemistry Offers 21st Century Packaging Materials, Raj. B. Durairaj, International Bottler & Packer, September 1999, pp. 34-38). However, the cost of these materials is too high and their large-scale use cannot therefore be recommended.

[0005] A further possibility might be to increase the thickness of the plastics bottle so as to reduce its permeability to atmospheric gases. However, this solution would also be too expensive and therefore not applicable.

[0006] The problem underlying the present invention is therefore to provide a low-cost material having a high degree of impermeability to oxygen.

[0007] It has surprisingly been found by the inventors in the present patent application that high-pressure treatment of a polymeric material substantially increases its impermeability to oxygen.

[0008] In particular, the above-mentioned problem is solved by a material and by a method for the production thereof as outlined in the appended claims.

[0009] The use of high isostatic pressures to sterilize foods enclosed in containers is known.

[0010] The term “polymeric material” according to the present invention is intended preferably to define a thermoplastic material and, more preferably, a single-layer or multi-layer, flexible or semi-rigid material composed of polymers, preferably selected from low-density and high-density polyethylene, polypropylene, PEN, EVOH, polyethylene terephthalate, copolymers and blends thereof, possibly additionally including nylon and/or scavangers. In the case of multi-layer materials, the layers are not generally of the same material, but are selected from materials of different types.

[0011] A particularly preferred thermoplastic material is a single layer of polyethylene terephthalate.

[0012] Naturally, the method for the production of a polymeric material with high degree of impermeability to oxygen according to the present invention is not limited to materials for containers but extends generally to materials of various types for different uses. Other polymeric materials which fall within the scope of the present invention are in fact materials having a substantially fibrous structure such as, for example, textile fibres. Other examples of suitable polymeric materials are so-called “non-woven fabrics”.

[0013] The term “containers” is intended to define containers of all types (including closure) which are made of a material, preferably a non-rigid material, that comprises at least one layer formed of polymeric material as defined above. Examples of these containers are beverage bottles of any shape and size, containers for preserving foods, packages of the Tetrapack® type, and containers of any type for holding liquids. The containers may be made purely of the polymeric material of the present invention or of a multi-layer material comprising, for example, in addition to the polymeric material according to the present invention, a card layer and/or an aluminium layer and/or a layer of plastics material which is not treated in accordance with the present invention.

[0014] For the purposes of the present invention, the “original” thermoplastic material (that is, the material produced by the polymerization process or resulting from the immediately subsequent extrusion process, for example, for the production of parisons) has preferably undergone a deformation outside its elastic range such as, for example, a blowing, stretching, pressing or injection-moulding process. This deformation outside the elastic range preferably follows a step for heating/softening the material.

[0015] The pressures applied to the thermoplastic material according to the present invention are pressures greater than 500 bar, preferably between 4000 and 9000 bar, even more preferably between 5000 and 7000 bar.

[0016] The high-pressure treatment according to the present invention is carried out by applying to the material temperatures generally of between −10° C. and 60° C., preferably between 10° C. and 30° C. Bearing in mind the increase in temperature caused by operating at high pressures, which is generally between 15° C. and 35° C. according to the pressure applied, the polymeric material will generally be subject to actual temperatures of between 0° C. and 80° C. during the process.

[0017] The high-pressure treatment may be carried out in known apparatus such as hyperbaric chambers which are filled with a fluid (for example, water or oil) and in which the pressure is raised to the desired pressure progressively over a predetermined period of time. The material is then subjected to the high pressure for a period of time which may vary from a few seconds to an hour or more.

[0018] Alternatively, the material may be subjected to a cycle of compressions-decompressions having an overall duration no greater than a few minutes, typically from 1 second to 2 minutes. Apparatus suitable for implementing the high-pressure process according to the present invention is that described in European patent application No. 99830254.1 of Apr. 4, 1999 in the name of the same applicant, of which the description relating to this apparatus is incorporated herein by reference. This patent application describes a hydrostatic device which can impart a pressure of up to 10,000 bar inside a pressurization chamber in which an individual container can be inserted. The pressurization chamber, on which a piston acts, is filled with water or other incompressible fluid. The pressure imparted by the piston is transmitted isostatically to the surface of the container and to the liquid held in the container. The piston is connected to a pressure-multiplying system which acts upstream. In particular, the piston is in communication with a chamber filled with an incompressible fluid which is acted on by a second piston the acting area of which is smaller than that of the first piston. The application of a small pressure to the second piston thus brings about a multiplication of the pressure up to a few thousand bar, by Pascal's principle, and this pressure can also be reached within an infinitesimal period of time.

[0019] The polymeric material may be treated at high pressures in any form, for example, in sheet form or already moulded in the form of bottles or other containers. Alternatively, the container may be subjected to the high-pressure treatment when it has already been filled with the beverage and sealed.

[0020] Permeability to Oxygen

[0021] The permeability to oxygen (the oxygen transmission rate) of the material treated at high pressures in accordance with the present invention has been determined in accordance with the ASTM F1307-90 standard which provides for the isostatic method and for the use of MoCon OX-TRAN 2/20 instrumentation operating, in this case, at 20° C. and 0% RH.

[0022] The test was performed on 333 ml PET bottles weighing 25 g.

[0023] The bottles were glued to a metal support to which two ⅛″ copper tubes with ⅛″ SWAGELOK connectors were welded. The sample thus assembled was connected to the inner half-cell of the apparatus. The carrier (nitrogen+2% hydrogen) was caused to flow inside the sample and the oxygen of the air which penetrated the system from the environment was carried to the coulometric sensor by the carrier.

[0024] The amount of oxygen which reached the detector through the bottle was assessed. The permeability of the bottle to oxygen was given by the difference in the signal produced by the detector, in stationary conditions, in the two stages:

[0025] (a-b) where

[0026] a=signal of sample+support+connectors

[0027] b=signal of the loop short-circuiting the connections of the inner cell;

[0028] b represents the base signal of the system, the sum of the instrument base and that due to the connections used.

[0029] The results of the test showed a gas barrier improvement of at least a 2 factor: sample (CC_(O2)/24h_(pkg.air)) permeability untreated bottle 0.038-0.043 bottle treated at 6000 bar 0.001-0.025

[0030] The data given show a net reduction in permeability to oxygen in the bottle treated at high pressures, in comparison with the untreated bottle. It should be noted that the test conditions used represent an extreme situation, since air was not present inside the bottle and the gradient of partial oxygen pressures between the outside and the inside of the bottle was therefore maximized. In the normal situation of use, that is, with a bottle incompletely filled with the beverage and sealed, the partial pressure of the gas inside the container will tend partially to resist the entry of further oxygen and the impermeability of the material to oxygen will be further increased thereby.

[0031] Permeability to Carbon Dioxide

[0032] The permeability to carbon dioxide (the CO₂ transmission rate) of the material treated at high pressures in accordance with the present invention has been determined by MoCon Permatran-C 4/40 instrumentation operating, in this case, at 20° C. and at ambient RH.

[0033] The test was performed on 330 ml PET bottles weighing 25 g.

[0034] Each bottle was introduced in a metallic chamber and glued to a metal support to which two ⅛″ copper tubes with ⅛″ SWAGELOK connectors were welded. The sample thus assembled was connected to the inner half-cell of the apparatus. The carrier (nitrogen) was caused to flow inside the sample and the dry CO₂ in the metallic chamber (100% CON). The carbon dioxide penetrated through the bottle wall was carried to the IR sensor by the carrier.

[0035] The permeability of the bottle to CO₂ was given by the difference in the signal produced by the detector, in stationary

[0036] conditions, in the two stages:

[0037] (a-b) where

[0038] a=signal of sample+support+connectors

[0039] b=signal of the loop short-circuiting the connections of the inner cell;

[0040] b represents the base signal of the system, the sum of the instrument base and that due to the connections used.

[0041] The results of, the test showed a gas barrier improvement of at least a 2 factor: sample (CC_(CO2)/24h_(pkg.100%CO2)) permeability untreated bottle 0.345-0.370 bottle treated at 6000 bar 0.075-0.170

[0042] Microscopic Structure of the Material

[0043] The modifications caused by the high-pressure treatment according to the present invention in the polymeric material were analyzed by electron microscope. The results are shown in the following drawings:

[0044]FIG. 1 shows an electron-microscope image of the structure of blown PET from a 333 ml bottle weighing 25 g,

[0045]FIG. 2 is an enlarged electron-microscope image of the blown PET structure of the bottle of FIG. 1,

[0046]FIG. 3 is an electron-microscope image, with the same enlargement as FIG. 1, of the blown PET structure of the bottle of FIG. 1 after treatment at 6000 bar.

[0047] As can be seen from the drawings, PET which has undergone deformation outside its elastic range, for example, by blowing, has a fibrous/laminar structure. As is clear from FIG. 2, there is a separation of a few microns between one fibre or lamina and another, to which the high degree of permeability of the material can substantially be attributed.

[0048] After high-pressure treatment, at 6000 bar in the specific case, the material had the structure shown in FIG. 3. The electron-microscope photograph shows a structure which is substantially compact or in any case with a compacted or reduced space between the fibres, so as to create a greater barrier to atmospheric oxygen for a given thickness and degree of stretch of the material.

[0049] It should in fact be noted that the permeability of PET to oxygen varies in dependence both on the thickness of the sheet and on the degree of deformation outside the elastic range which it has undergone. These parameters in turn depend on parison weight and on bottle size; for a given parison weight, the material will be thinner and more deformed the larger is the volume of the bottle blown; conversely, for a given bottle size, the degree of deformation of the material will be greater for parisons of small weight and length.

[0050] Naturally, a person skilled in the art will be able to adapt the above-described method to particular requirements such as, for example, those resulting from its application to materials of different kinds, without thereby departing from the scope of the present invention. 

1. A method for the preparation of polymeric material with increased impermeability to gases, particularly to oxygen, the meth comprising the step of subjecting the polymeric material to hydrostatic pressure.
 2. A Method according to claim 1 in which the hydrostatic pressure step is carried out at a pressure greater than 500 bar.
 3. A method according to claim 1 in which the hydrostatic pressure step is carried out at a pressure of between 4000 bar and 9000 bar.
 4. A method according to claim 1 in which the hydrostatic pressure step is carried out at a pressure of between 5000 bar and 7000 bar.
 5. A method according to any one of claims 1 to 4 in which temperatures of between −10° C. and 60° C. are applied to the polymeric material.
 6. A meth according to claim 5 in which temperatures of between 10° C. and 30° C. are applied to the material.
 7. A method according to any one of claims 1 to 6 in which the polymeric material reaches temperatures of between 0° C. and 80° C.
 8. A method according to any one of claims 1 to 7 in which the hydrostatic pressure treatment has a duration of between 1 second and 2 minutes.
 9. A method according to any one of claims 1 to 8, the method comprising the steps of: a) inserting the polymeric material in a chamber for treatment at hydrostatic pressure, b) filling the chamber for treatment at hydrostatic pressure with a substantially incompressible fluid such as water or oil, c) applying a predetermined temperature to the substantially incompressible fluid, d) raising the pressure in the chamber for treatment at hydrostatic pressure to a predetermined level and maintaining this hydrostatic pressure for a predetermined period of time, e) reducing the pressure to atmospheric pressure, removing the substantially incompressible fluid, and extracting the polymeric material.
 10. A method according to claim 9 in which the hydrostatic pressure treatment is implemented by means of a cycle of compressions/decompressions.
 11. A method according to any one of claims 1 to 10 in which the polymeric material is a thermoplastic material, a synthetic fibre, or a non-woven fabric.
 12. A method according to claim 11 in which the thermoplastic material is a single-layer or multi-layer, flexible or semi-rigid material closed of polymers, preferably selected from low-density and high-density polyethylene, polypropylene, PEN, EVOH, polyethylene terephthalate, copolymers and blends thereof, possibly additionally including nylon and/or scavangers and, most preferably, is a single layer of polyethlene terephthalate.
 13. A method according to claim 11 or claim 12 in which the polymeric material has previously undergone a deformation outside its elastic range.
 14. A method according to claim 13 in which the polymeric material has undergone softening by heating immediately prior to the deformation outside its elastic range.
 15. A method according to claim 13 or claim 14 in which the deformation outside the elastic range is a deformation caused by a blowing, stretching, pressing, or injection-moulding process.
 16. A polymeric material obtainable by the method outlined in claims 1 to 15, wherein thi said polymeric material previously undergone a deformation outside its elastic range and has then be subjected to a hydrostatic pressure treatment at a pressure greater that 500 bar.
 17. A polymeric material according to claim 16 in which the material has a microscopic structure which is substantially compact or with compacted or reduced spaces between the fibres.
 18. A polymeric material according to claim 16 or claim 17 in which the material is a thermoplastic material, a synthetic fibre, or a non-woven fabric.
 19. A polymeric material according to claim 18 in which the thermoplastic material is a single-layer or multi-layer, flexible or semi-rigid material composed of polymers, preferably selected from low-density and high-density polyethylene, polypropylene, PEN, EVOH, polyethylene terephthalate, copolymers and blends thereof, possibly additionally including nylon and/or scavangers and, most preferably, is a single layer of polyethylene terephthalate.
 20. A polymeric material according to any one of claims 16 to 19, in which the polymeric material has undergone softening by heating immediately prior to the deformation outside its elastic range.
 21. An article comprising at least one layer of the polymeric material according to any one of claims 16 to
 20. 22. An article according to claim 21, the article being a container.
 23. An article according to claim 22 in which the container is a container for liquids, in particular, a bottle including the closure. 